Human granulocytic ehrlichiosis is an emerging infectious disease caused by the tick-borne pathogen Anaplasma phagocytophila. A. phagocytophila is an obligate intracellular pathogen with tropism for neutrophils. The mechanism by which this organism survives within this hostile environment remains to be fully elucidated. One survival mechanism utilized by this pathogen may be the result of inhibiting the potent antimicrobials generated by the oxidative burst. A. phagocytophila inhibition of the oxidative burst has been correlated with the down-regulation of gp91phox, an integral component of NADPH oxidase, the facilitator of oxidative burst. The manner of transcriptional inhibition is unknown. I propose that A. phagocytophila inhibits gp91phox expression by modulating the activators and repressor that regulate expression of this gene. My preliminary data reveal that there may be several facets to A. phagocytophila's effect on the expression of gp91 phox. Infection results in enhanced interaction of the repressor with the promoter of gp91phox. CDP binds to 6 separate sites within the promoter of gp91phox. Infection enhanced the interaction of CDP with all sites tested. Infection also resulted in tyrosine phosphorylation of a protein with a molecular weight similar to that of CDP. In addition, transcription of lactoferrin, a second CDP regulated gene, is also down-regulated. The repressor was not the only protein to be affected by infection. Protein concentrations of IRF-1 and PU 1, two transcription activators which regulate the expression of gp91phox were modulated during infection. IRF-1 was affected at the transcription level while PU.1 was affected at the protein level. Based on these results, I hypothesize that A. phagocytophila modulates the expression of gp91 phox by targeting the proteins that interact with the promoter. Aim 1. To determine the effect of A. phagocytophila infection on CCAAT displacement protein (CDP), a major repressor of gp91phox. Aim 2. To define the effect of A. phagocytophila infection on the activity of the activator proteins and their binding capacity of the gp91phox promoter.